The present invention relates to optical scanners and in particular to a laser probe that uses scatterometry, for example, to detect defects on the surface of a memory disk.
A computer hard disk drive comprises a magnetic or optical memory disk mounted on a spindle, which is driven by a motor to rotate the magnetic disk at high speed. A read/write head, kept in close proximity to the surface of the rotating magnetic disk, reads or writes data on the magnetic disk. The read/write head is separated from the surface of the magnetic disk by an air bearing created by the high-speed rotation of the magnetic disk. The read/write head flies on this air bearing, e.g., at a height of approximately one microinch. The closer the read/write head is to the surface of the magnetic disk, the more information may be written on the disk. Thus, it is desirable for the read/write head to fly as close as possible to the surface of the magnetic disk.
Typical magnetic disks comprise an Al substrate, a NiP layer which is plated on the Al, polished and then textured, an underlayer (e.g. Cr or NiP) sputtered on the plated NiP layer, a thin film of magnetic recording material (typically a Co alloy) sputtered on the underlayer, a protective overcoat sputtered on the magnetic film and a lubrication layer formed on the overcoat. Magnetic disk manufacturing specifications typically require that asperities and depressions on a magnetic disk are smaller than a certain size. Although magnetic disks are typically textured to have a specified roughness, there has been a trend in the industry to make magnetic disks smoother and smoother. Presently, some magnetic disks are specified to have a roughness less than or equal to about 10 xc3x85 (1 nm). As the specified roughness is decreased, the size of the asperity or depression that can be tolerated is decreased.
The precision with which the read/write head flies over the magnetic disk requires that care is taken during manufacturing to assure that there are no protrusions or asperities on the disk surface that may interfere with the read/write head. A protrusion on the surface of the disk that contacts the read/write head during use may damage the head or the disk.
Accordingly, during manufacturing of magnetic or magnetic-optical disks, tests are performed with xe2x80x9cmedia certifiersxe2x80x9d using, e.g., glide heads, to ensure that there are no defects, such as asperities, voids or contamination, that might interfere with the read/write head. Accurate testing of disks for such defects assures that the disk manufacturer does not unnecessarily reject good quality disks or pass on poor quality disks that may later fail.
Certifying disks using glide heads can be a time consuming task. Each disk must be individually mounted on a spindle. The disk is rotated at high speed, while a burnish head is moved across the surface to remove loose debris and then a glide head is moved across the surface of the disk to check for asperities or defects. The disk is then dismounted from the spindle. If the disk is found to have an unacceptable defect, the disk is rejected. Typically, however, before rejecting the disk, the disk is retested. A different media certifier is sometimes used to retest the disk, which requires mounting the disk on a different spindle, rotating the disk while burnish and glide heads move across the surface and dismounting the disk. Consequently, a disproportionate amount of time may be spent retesting a defective disk.
A laser scatterometer, in accordance with the present invention, may be used to detect defects on objects such as memory media. The laser scatterometer includes a light source that produces a light beam that is incident on the object being tested. The light reflected from the object has two components, a specular component and a scattered component, which is caused, e.g., by defects. A photodetector, which receives the reflected light, includes a light detector (such as a photodiode) and a beam block. The beam block is adjustable so that the specular component can be blocked while minimizing interference with the scattered component. In one embodiment, the beam block is masked on the lens of the light detector and the entire photodetector is moved so that the specular light is incident on the beam block. Because the beam block can be finely adjusted to block only the specular light, the beam block may be adjusted to permit more small angle scattered light to pass to the light detector than conventional systems. The light source, such as a laser or laser diode, produces a beam of light that is focused so as to minimize the spot size at the beam block while maximizing the spot size on the surface of the object being tested. Consequently, the object may be tested quickly and the small angle scattered light is maximized.
Because a large spot size is used at the surface of the object, e.g., the rotating disk, the laser scatterometer may be used at the same time as the burnishing process, immediately before glide testing. The large spot size is used to detect large defects. Moreover, because the small angle scattered light signal is maximized, defects (e.g., sub-bumps) that are conventionally detected only with glide testing, may be detected with the laser scatterometer. By using the laser scatterometer during the burnishing process, disks with large defects may be immediately rejected without requiring additional time-intensive testing of the defective disk. The remaining disks, which were not rejected, can then undergo additional testing, e.g., glide testing, for smaller defects. Consequently, eliminating clearly defective disks by using the laser scatterometer during the burnishing process will increase the throughput in the glide testing process and eliminate the time that would have been required to test these defective disks.